ServiceWorker: Change worker script fetch error code.

Change layout test error code, add worker script redirect test.

(1/3): https://codereview.chromium.org/546353003/
(2/3): https://codereview.chromium.org/567903002/
(3/3): this

BUG=408421
TEST=http/tests/serviceworker

Review URL: https://codereview.chromium.org/597483002

git-svn-id: svn://svn.chromium.org/blink/trunk@182570 bbb929c8-8fbe-4397-9dbb-9b2b20218538